This invention relates to generally to bipolar differential amplifiers, and particularly to such amplifiers where linear performance, i.e., producing a differential output that linearly tracks an input, is desired.
In the field of this invention it is known that many applications such as broadband radio frequency (RF) input stages require to be very linear with high gain and low noise.
A standard bipolar differential amplifier (sometimes called a long-tailed pair) has a tanh (hyperbolic tan) transfer characteristic, which means that there is only a small range of differential input voltage in which the differential output signal tracks the input in a linear fashion.
Several approaches are known for improving the linearity of such a bipolar differential amplifier:
1) Emitter degeneration by adding a resistance R between the two transistor emitter electrodes. This will give a linear range of +/xe2x88x92(I*R), where I is the current source load on each emitter electrode. A consequence of this emitter degradation approach is very high current if the gain is to be kept high by making R small. Also, the noise figure is worsened.
2) Putting in parallel several differential amplifiers with varying input DC offsets, usually by changing the emitter size of one of each transistor pair. This approach gives a limited improvement.
There is therefore a need for a bipolar differential amplifier wherein the above mentioned disadvantages may be alleviated.